Usage CONtrol (UCON) model is the latest major enhance-
ment of the traditional access control models which enables
mutability of subject and object attributes, and continuity of
control on usage of resources. In UCON, access permission
decision is based on three factors: authorisations, obliga-
tions and conditions. While authorisations and obligations
are requirements that must be ful lled by the subject and
the object, conditions are subject and object independent
requirements that must be satis ed by the environment. As
a consequence, access permission may be revoked (and the
access stopped) as a result of changes in the environment
regardless of whether the authorisations and obligations re-
quirements are met. This constitutes a major shortcoming
of the UCON model in pervasive computing systems which
constantly strive to adapt to environmental changes so as to
minimise disruptions to the user. To overcome this limita-
tion, this paper proposes a Context-Aware Usage CONtrol
(CA-UCON) model which extends the traditional UCON
model to enable adaptation to environmental changes in the
aim of preserving continuity of access. When the authori-
sations and obligations requirements are met by the subject
and the object, and the conditions requirements fail due
to changes in the environment or the system context, CA-
UCON model triggers speci c actions to adapt to the new
situation. Besides the data protection, CA-UCON model
so enhances the quality of services, striving to keep explicit
interactions with the user at a minimum.